1. Field of the Invention
This invention relates to a method and apparatus for effecting thermal printing, by utilizing a heat sensitive printing medium, and more particularly to a method and apparatus for thermal printing wherein nonuniform heating of heat generating members caused by their unequal registance values is automatically compensated for.
2. Description of the Prior Art
Generally, a thermal head of heat sensitive printing head comprises a plurality of heat generating resistors corresponding to the number of picture elements in the scanning direction of a recording medium. For example, where a picture information is printed on a printing paper having a size of the Japanese Industrial Standard, Series A, No. 4, 1728 resistors are used, whereas where a picture information is printed on a printing paper having a size of the Japanese Industrial Standard, Series B, No. 4, 2048 resistors are used. Electric power corresponding to the picture information is supplied to respective heat generating resistors to cause them to generate heat. Accordingly, when the heat sensitive printing medium is brought into slide contact with the thermal head, a portion of the printing medium corresponding to an energized heat generating resistor generates heat to change color. When the heat sensitive printing medium comprises an ink donor film, the ink applied thereon melts or evaporates to reproduce a picture image corresponding to the picture information.
Where the heat generating resistors of the thermal heat are made of thick film type heat generating elements, their resistance values usually differs by 20-30%. Even when the heat generating resistors are made of thin film type heat generating elements their resistance values differ by 5-20%.
Thus, inequality of the resistance values of respective heat generating resistors of the thermal head is inevitable so that the quality of the printed picture image is not excellent due to nonuniform tone. More particularly, where the thermal head is driven with constant voltage, the quantity of heat generated by each heat generating resistor is proportional to V.sup.2 /R, where V represents the impressed voltage and R the resistance value of each heat generating resistor. Where constant current flows through the thermal head, the quantity of heat generated by each heat generating resistor is proportional to I.sup.2 R, where I represents the current, and R the resistance value of each heat generating resistor. As above described, the quantity of heat generated by each heat generating resistor, or the tone of the printed picture image is influenced directly by its resistance value. Accordingly, when the resistance values R of respective resistors are not equal, the tone of the printed picture image is not uniform.
Recently, it has been desired that the printed picture image should have a high density as well as a high picture quality. Nonuniformity of the resistance values of the resistors not only degrades the quality of the printed picture image, but also decreases the yield of satisfactory resistors which results in the increase in the cost of manufacturing the resistors.